1. Field of the Invention
The present invention relates to a device for treating vaporized fuel generated in a fuel supply system of an internal combustion engine not only when the fuel supply system is being filled with fuel but also when such a fuel filling operation is not underway.
2. Description of the Related Art
It is well known to temporarily hold an amount of vaporized fuel in an internal combustion engine, in a canister provided with an activated charcoal layer therein, and to later re-introduce the vaporized fuel to the engine. In a known device, fuel vapor from a fuel tank can be captured by the canister while the filler cap covers the fuel inlet to the fuel tank, i.e., a fuel filling operation is not carried out, and to obtain a predetermined positive pressure in the fuel tank, the canister is provided with a check valve. This known canister system can not prevent fuel from being exhausted to the atmosphere when the filler cap is removed, and therefore, it is necessary to provide a method of preventing the vaporized fuel from being exhausted to the atmosphere when the filler cap is removed to enable a fuel filling operation to be carried out.
Accordingly, a system has been proposed wherein two canisters are provided, and these canisters are switched by a switching means so that one of the canisters is used for holding vaporized fuel generated when the filler cap is removed, i.e., when a fuel filling operation is carried out, and the other canister is used for holding vaporized fuel generated when the filler cap is attached, i.e., a fuel filling operation is not carried out. See Japanese Unexamined Utility Model Publication No. 59-14460 and Japanese Unexamined Utility Model Publication No. 61-141129.)
In such a device provided with a pair of canisters, the provision of an inlet pipe of a fuel tank, which extends downward to a point adjacent to the bottom inner surface of the fuel tank, has been proposed to prevent a direct impingement of incoming fuel on the fuel in the fuel tank during the fuel filling operation; as this would produce bubbles in the fuel and these bubbles exhausted to the outside atmosphere. The downwardly elongated inlet tube also serves to reduce the total surface area of the fuel in the fuel tank which comes into contact with the atmospheric air.
Such a canister device, however, can hold only a limited amount of vaporized fuel, and therefore, it is preferable to reduce the amount of vaporized fuel from the fuel tank, to maintain the canister device in an activated condition. Therefore, a check valve is provided to ensure a constant pressure inside the fuel tank when the filler cap covers the fuel tank inlet which allows the amount of vaporized fuel in the fuel tank to be suppressed or lowered. Nevertheless, the high pressure in the fuel tank causes a large amount of vaporized fuel to be instantly exhausted to the atmosphere when the filler cap is removed for a fuel filling operation, and to obviate this difficulty, it is necessary to maintain the pressure inside the fuel tank at a lower value. This, however, causes the amount of vaporized fuel to be increased, and thus a drawback arises in that the canister must have an increased volume to cope with the increased amount of vaporized fuel.